The present invention relates to an improved hollow core composite bushing and, more particularly, to an improved bushing for use within a dead tank circuit breaker.
Generally, dead tank circuit breakers are provided to direct the unimpeded flow of current in a network under normal operating conditions, and to interrupt the flow of excessive current in a failing network. Dead tank circuit breakers also may interrupt capacitor banks, isolate transformer switching, interrupt load current and/or perform a high speed open-close-open sequence to clear a fault. The successful achievement of these functions generally is dependent upon the circuit breaker's specific mechanical design to meet the demands of opening and closing the circuit breaker contacts, and dependent upon the specific electrical design to ensure that the circuit breaker can withstand the electrical stresses applied to it.
During the opening sequences an electric arc forms between the contacts of the circuit breaker. The discharge is utilized to assist in circuit-interruption. For example, in an AC network, the arc is tolerated in a controlled manner until the occurrence of a natural current zero of the sine wave. The arc then is rapidly quenched to limit the reaction of the network stability to the interruption. A typical circuit breaker may utilize the gas Sulfur Hexafluoride (SF.sub.6) as the arc-quenching media. The gas SF.sub.6 is preferred due its superior electrical insulation qualities and its ability to recover quickly (e.g., within 3 .mu.sec) from high temperature arc interruption (e.g., above 2,000.degree. K). Pure SF.sub.6 gas is colorless, odorless, tasteless, nontoxic, chemically stable and non-flammable. SF.sub.6 is in its gaseous state at 20.degree. C. and atmospheric pressure, and has a density of approximately 4.7 times that of air. The critical temperature of SF.sub.6 is 45.6.degree. C., and may liquified under pressure.
A dead tank circuit breaker assembly consists of six isolation insulators for a 3-phase system. Three insulators are connected to a power source on one side of three interrupters and direct the 3-phase voltage through the circuit breaker. The 3-phase voltage is directed out by another set of 3 isolation insulators, which are on the load side of the circuit breaker interrupters.
Dead tank circuit breakers provide for separate internal transition shields that are secured to a grounded enclosure as a separate piece. Referring to the illustration shown in FIG. 1, a hollow core bushing 10 is placed over the transition shield 20 and secured to the grounded enclosure 22. This configuration requires the internal diameter of the hollow core bushing to be larger than the diameter of the transition shield, with sufficient clearance to prevent mechanical contact. The transition shield must also have an inside diameter sufficient to accommodate the dielectric constant (permittivity) between it (i.e., ground) and the conductor 24 (i.e., the electric potential). These factors contribute to larger parts. FIG. 2 illustrates a cut-out portion of the hollow core bushing shown in FIG. 1. As shown in FIG. 2, a space is formed between hollow core bushing 10 and transition shield 20 (also shown in FIG. 1). Conductor 24 is not shown in FIG. 2 for convenience.
Porcelain and composite bushings that are currently used within dead tank circuit breakers are problematic due to their large diameters and weight. Such bushings are fairly expensive and timely to manufacture and add to the overall weight and expense to dead tank circuit breakers. Moreover, porcelain bushings are susceptible to catastrophic failure when exposed to high impact which in turn may result in substantial damage to external and nearby equipment, as well as serious human injury.
It is therefore an object of this invention to provide an improved hollow core composite bushing that overcomes the above-mentioned disadvantages of existing bushings.
It is another object of this invention to provide an improved hollow core composite bushing with a reduced diameter as compared to previous designs.
It is a further object of this invention to provide a hollow core bushing with a lower center of gravity.
It is an additional object of this invention to provide a hollow core bushing having a reduced weight as compared to previous designs.
Various other objects, advantages and features of the present invention will become readily apparent to those of ordinary skill in the art, and the novel features will be particularly pointed out in the appended claims.